THE NERVOUS SYSTEM 1421 



in four groups or stalks — frontal, parietal, occipital, and inferior or ventral, 

 (a) The fibres of the frontal stalk, having emerged from the front part of the 

 external surface, traverse the anterior limb of the internal capsule, and most 

 of them pass to the cortex of the frontal lobe. Some of these fibres are 

 thalamo-caudate and thalamo-lenticular, as regards their destination. 

 {b) The parietal stalk, having issued from the external surface of the optic 

 thalamus, passes for the most part through the internal capsule, but also, 

 to a certain extent, through the external capsule, to the cortex of the parietal 

 lobe, and the Rolandic region of the frontal lobe, (c) The occipital stalk 

 issues from the outer part of the pulvinar, and, having traversed the postlen- 

 ticular portion of the posterior limb of the internal capsule, it passes back- 

 wards and outwards, external to the posterior comu of the lateral ventricle, 

 and so reaches the cortex of the occipital lobe, (d) The inferior or ventral 

 stalk emerges from the front part of the inferior surface of the optic thalamus, 

 and its fibres arise as the axons of the cells of the outer and inner nuclei. 

 The most superficial of these fibres constitute a band, called the ansa lenti- 

 cularis, which enters the nucleus lenticulcuris, where it terminates. The 

 remaining fibres pass outwards beneath the nucleus lenticularis to the cortex 

 of the temporal lobe and island of Reil. 



Besides the thalamo-cortical fibres there are eortlco-thalamic fibres, which 

 pass from the various parts of the cerebral cortex into the optic thalamus, 

 where they terminate in arborizations around the thalamic cells. 



Development. — The optic thalamus is developed as a thickening of the 

 lateral w^all of the thalamencephalon or diencephalon. 



Subthalamic Tegmental Region. — This region represents the 

 upward prolongation of the tegmental fibres of the cms cerebri 

 beneath the posterior portion of the optic thalamus. The parts 

 to be noted are as follows : upward prolongations of the red nucleus 

 and substantia nigra of the tegmentum of the cms cerebri; the 

 mesial fillet ; the fibres of the superior peduncle of the cerebellum ; 

 and the corpus subthalamicum, or nucleus of Luys. The red 

 rMcleus and the substantia nigra gradually disappear, and are no 

 longer visible at the level of the corpus albicans. The mesial 

 fillet lies on the superficial and outer aspects of the red nucleus. 

 The fibres of the superior pedimcle of the cerebellum partly ter- 

 minate in connection with the cells of the red nucleus, but many 

 of them surround it in the form of a capsule. Beyond the red 

 nucleus the mesial fillet, fibres of the superior cerebellar peduncle, 

 and fibres which issue from the red nucleus enter the inferior sur- 

 face of the optic thalamus, and terminate in connection with the 

 thalamic cells. Some of these fibres may pass through the optic 

 thalamus into the internal capsule, and thence to the cortex of the 

 Rolandic region of the cerebral hemisphere. The corpus sub- 

 thalamicum or nucleus of Luys is a small lenticular mass of grey 

 matter, surrounded by white fibres, which lies on the outer side 

 of the substantia nigra. 



Epithalamus. — The epithalamus includes the following parts: 



1. Pineal body. 3. Trigonum habenulae. 



2. Strise medullares. 4. Posterior commissure. 



Pineal Body, or Epiphysis Cerebri. — The pineal body is so named 

 because it resembles a pine-cone. It is situated on the dorsal or 

 superior surface of the mesencephalon, and occupies the depression 

 between the upper quadrigeminal bodies. It is of small size, dark 



